1. Technical Field
The present disclosure relates to a method of stacking a plurality of dies to form a stacked semiconductor device or system, and to the stacked semiconductor device or system.
2. Description of the Related Art
Currently assembly of a plurality of dies, where one of them has a thickness equal to, or lower than, 50 μm, includes a specific technique in order not to damage the thin die and allow die attach film (DAF) tape separation. Due to the wafer sawing process, such a thin wafer is singulated by means of a “Dicing Before Grinding” (DBG) process. This leaves singulated dies face down on a grinding tape. The same dies, then, are further laminated onto either a mounting tape or onto a DAF tape and a mounting tape to be processed at die attach.
In case of using a mounting tape only, a DAF tape is pre-applied, with a tape cut-and-place process, to the thicker die. The same process is not easily adaptable to the thinner die.
In the case where the singulated die is laminated onto a DAF tape, the latter is in some way separated at the sawing kerfs. Two methods available are either laser DAF cutting or low temperature expansion of the DAF tape which causes a rupture of the DAF tape along the existing kerf lines.
The process of tape cut-and-place utilizes additional hardware options on the die attach machines and is limited in application to large dies, due to economic reasons and availability of tape reels of the required width, as well as the manipulation of reduced tape sizes.
The method of DAF laser cutting is applicable to any die size. The limitations of this method are mainly due to high costs and low process throughput.
The method of DAF expansion at low temperatures utilizes dedicated equipment. To date this method is limited to large die sizes and is not applicable to die sizes less than 3×3 mm. Moreover the process is not isotropic and the DAF tape breakages are inconsistent across the wafer axes.
There is an ever present need to reduce package dimensions particularly the package height by the consumer market. Die thickness reduction is a critical element to achieve. A grinding process to achieve thin dies as far as 20 μm is available. Accordingly, there is a desire for achieving a cost effective way to perform die attach for such thin dies. In particular, there is a desire to achieve a cost effective and reliable method to stack a thin die on top of a MEMS sensor.